What is the ruling of Islamic law regarding a divorced woman accepting marriage to a man who was the cause of her divorce, knowing that he is married with children and his wife is pregnant, and that she (the divorced woman) fulfilled her duties towards her former husband perfectly despite his mistreatment?

You must repent to Allah for having a relationship with a married man and mend your relationship with your first husband by admitting your mistake and returning to your home and children. You can seek help from trustworthy people. If attempts at reconciliation fail, it is not advisable to rush into marrying the other man. Scholars have differed on the validity of a woman's marriage to the one who corrupted her against her husband. Moreover, his circumstances are not ideal, and he might repeat his actions with others. If you are still in your iddah (waiting period), he is not allowed to propose to you, and it is not permissible for you to accept. It is recommended to completely sever ties with this man, even if you do not return to your first husband. You must think wisely and not succumb to emotions.